While maternal substance use problems increase the likelihood of behavior problems in children, child outcomes are varied, leading to interest in understanding additional family factors that contribute to the development of behavior problems in children impacted by maternal substance abuse. The purpose of this study is to examine harsh parenting and family conflict as potential moderators of the relationship between symptoms of maternal substance use problems and child externalizing behavior problems. The non-clinical sample for this study included 250 low-income parents whose preschool age children were enrolled in Head Start programs in a Southern state. This study utilized data collected during two home visits, an average of 10 months apart, with data on family functioning and maternal symptoms of substance use problems collected at the first time point and child externalizing behavior collected at the second time point. Over one-third of the children (38.1 %) had clinically elevated externalizing behavior scores. We used regression analysis to examine whether harsh parenting or family conflict moderated the relationship between maternal substance use symptoms and child externalizing behavior. In this community sample, we found that in the absence of family risks related to harsh parenting and family conflict, maternal symptoms of substance use problems did not have a significant impact on child externalizing behavior in preschool children. However, when high levels of family conflict or harsh parenting were present, symptoms of maternal substance use problems increased the risk of externalizing behavior problems in children.